1. Field of the Invention
This invention relates to high-durable pneumatic radial tires, and more particularly to a pneumatic radial tire having considerably improved tire reinforcing properties by improving an internal structure of steel filament in steel cords as a tire reinforcement to thereby enhance the corrosion fatigue resistance of the steel filament itself.
2. Description of the Prior Art
When steel cords are used as a material for a carcass ply or a belt of a tire, various performance characteristics required for the tire deteriorate by a cord-breaking-up (CBU) failure mainly resulting from corrosion fatigue failure of the carcass, or by a corrosion of cords in the belt layers, particularly an outermost belt layer produced from cracks penetrating through a tread and a fatigue failure accompanied therewith, so that the use of steel cords comes into question in view of the tire durable life.
In this connection, the inventors have previously confirmed that the corrosion fatigue resistance of steel filaments used as a material for the steel cord can be improved by making an interlamellar distance between cementites in pearlite structure of the steel filament larger than the usual one of the prior art, which is disclosed in Japanese Patent Application No. 57-195,332.
Further, the inventors have examined with respect to the influence of grain size in pearlite crystals forming the pearlite structure and found out that the properties of the steel cord can considerably be improved by the particularization of the grain size of pearlite crystals.
In the steel cord, the progress of cracks based on the corrosion fatigue of the steel filament is governed by a stress state at an end portion of cementite inside the pearlite structure and a shearing stress state between cementite and ferrite. Although the average value of interlamellar distance between cementites in pearlite structure according to the prior art was usually 200-250 .ANG. when it is increased to a range of 300-500 .ANG. by optimizing a heat treatment before a final drawing at a production step of steel cords, the above mentioned stresses can be mitigated to make the growth and progress of cracks slow.
The cracks advance not only through the end portion of cementite in a pearlite crystal grain but also through the grain boundary in pearlite structure. In the latter case, it has been elucidated that the advance of cracks through pearlite crystal grains can be suppressed by making the grain boundary into a partition wall.
Namely, it is recognized that the fine division of pearlite crystal grain is required together with the increase of interlamellar distance.